tree-optimization/110742 - fix latent issue with permuting existing vectors

tree-optimization/110742 - fix latent issue with permuting existing vectors

When we materialize a layout we push edge permutes to constant/external
defs without checking we can actually do so.  For externals defined
by vector stmts rather than scalar components we can't.

	PR tree-optimization/110742
	* tree-vect-slp.cc (vect_optimize_slp_pass::get_result_with_layout):
	Do not materialize an edge permutation in an external node with
	vector defs.
	(vect_slp_analyze_node_operations_1): Guard purely internal
	nodes better.

	* g++.dg/torture/pr110742.C: New testcase.